The goals of this project are to understand the nature and significance of pseurouridine biosynthesis in mammalian transfer RNAs and to define and characterize the tyrosine transfer RNAs of mammalian cells. Tyrosine transfer RNAs will be isolated from normal mammalian tissues and from transformed cells. The structures of these isoaccepting transfer RNAs will be analyzed and compared. Analyses of pseurouridine biosynthesis will include purification of the relevant enzymes from representative sources, examination of the molecular and catalytic properties of these enzymes, and assessment of their roles in eukaryotic nucleic acid metabolism.